A hydrodynamic bearing unit is used for a small motor, for example, a spindle motor for a disk drive apparatus, a polygon scanning motor for a laser beam printer (LBP) and the like. There are demands for the above various kinds of motors with higher rotational accuracy, enhanced speed, reduced costs, reduced noise, etc. One of the components which determine these required performances is a bearing supporting the spindle of such a motor. In recent years, hydrodynamic bearings excellent in the required performances such as above are studied or actually used.
For example, in a spindle motor for storage disk drive apparatuses such as hard disks, a hydrodynamic bearing unit comprising a radial bearing portion and a thrust bearing portion is used. The radial portion supports a rotational member in a non-contact manner in the radial direction by the hydrodynamic effect of the lubricating fluid which occurs in a radial bearing gap between the rotational member and the bearing sleeve, and a thrust bearing portion supports the rotational member in a non-contact manner in the thrust direction by the hydrodynamic effect of the lubricating fluid which occurs in a thrust bearing gap between the housing and the rotational member (e.g., refer to Japanese Unexamined Patent Publication No. 2000-291648).
Since storage disk drive apparatuses such as the above hard disks are used in a relatively wide temperature range, a lubricating fluid having low evaporation rate and low viscosity is suitable for hydrodynamic bearing units used in the spindle motors of the above storage disk drive apparatuses. For example, ester-based lubricating oils are used (e.g., refer to Japanese Unexamined Patent Publication No. 2003-172336).